The present disclosure relates to a method and an arrangement for monitoring the voltage on electrical storage units, to a battery and to a motor vehicle having such a battery that are able to be used particularly for improved establishment of overvoltages and/or undervoltages on modules of the electrical storage unit.
For the supply of power to electric drives in electric and hybrid vehicles, high-voltage lithium ion batteries are frequently used. The chemistry of these batteries means that they have a hazard potential—if operating limits are exceeded, there may be a battery fire or leakage of dangerous chemical substances.
It is a task of the battery management system (BMS) to regulate the charging and discharge of the battery under the given requirements such that safety is ensured. To this end, appropriate algorithms and application parameters are stored in the BMS.
Possible hazard triggers in the case of lithium ion cells are overvoltage (e.g. voltages above 4.2 V) and undervoltage (e.g. 2.5 V) or recharging following an undervoltage. The voltage limits are accordingly monitored by the BMS and the transition to the safe state is initiated in the event of overvoltage or undervoltage.
Both safety (an overvoltage or undervoltage must be identified) and availability (an overvoltage or undervoltage should not be identified erroneously) require the most precise possible measurement of the voltage.
However, the measured voltage when measuring cell voltage comprises two contributing elements: 1) the source or idle voltage on the cell: U_OCV and 2) the current-dependent voltage on the nonreactive resistances inside and outside the cell (e.g. as a result of contact points): U_ohm.
The following relationship is obtained for the measured voltage U_measured:U_measured=U_OCV−U_ohm.
In respect of safety or cell loading, only the idle voltage U_OCV on the cell is relevant. Therefore, the nonreactive influences can result in 1) an overvoltage not being identified or 2) an undervoltage being identified erroneously.
In ordinary battery controllers, the monitoring of the voltage is implemented by hardware elements, e.g. by comparators, which perform this function either independently or as support or a fallback level for the software monitoring. In this case, this hardware monitoring is usually limited to just one input parameter—the voltage itself—on account of the circuit complexity. The current as a further parameter, as a cause of the voltage drop, therefore cannot be taken into account additionally.
Moreover, the nonreactive resistance varies over the lifespan on account of cell ageing, corrosion or deteriorating contact-connection on measuring lines and at contact points. These effects cannot be quantified by the conventional monitoring.
The prior art, for example DE 102010041492 A1, also discloses the practice of monitoring traction batteries by dispensing with the “hardware path” for monitoring safety-related variables and by monitoring measure variables such as voltage or temperature using a microcontroller on which safety functions are implemented by software. To ensure the redundancy of the monitoring, this solution involves the communication link between the battery and the microcontroller being monitored and checked for plausibility.